The influence of heterogeneous seafloor heat flux on the cooling patterns of Ganymede’s and Titan’s subsurface oceans

Abstract

Several icy moons of Jupiter and Saturn are known to possess deep water oceans. Heating in the rocky mantle underneath often produces heterogeneous heat flux patterns at the ocean’s seafloor. How this internal ocean dynamically relates the seafloor to the surface ice shell is a crucial question to understand the long term evolution of icy moons. Here we investigate how a heterogeneous seafloor heat flux pattern affects the convection and heat transfer in the subsurface ocean of large icy worlds involving a high pressure ice layer beneath the seafloor such as Titan or Ganymede. We perform rotating convection simulations in a thin 3D spherical shell with a prescribed heterogeneous bottom heat flux inferred from 3D convection simulations of the underlying mantle (Choblet et al., 2017b). In our simulations, although the amplitude of imposed inner boundary heat flux heterogeneity is rather moderate, preferred longitudes of intense outer boundary heat flux are highly correlated with longitudes of intense inner boundary heat flux. In addition, a small imposed inner boundary large-scale order 2 pattern is amplified at the outer boundary heat flux by the convection in the thin shell. Lastly, deviations from axisymmetry and equatorial symmetry in the outer boundary heat flux increase with the main convection vigor and the amplitude of the inner boundary heterogeneity. In our models polar vs. equatorial cooling is mostly controlled by inertial effects, as was found by Amit et al. (2020) for homogeneous boundary conditions, with the latitudinally equilibrated inner boundary heterogeneity acting to reduce the amplitude of this effect. Our results support polar cooling for Titan’s sub-surface ocean.